In an image forming device such as an electrophotographic device and an electrostatic recording device, first, an electrostatic latent image famed on the photoconductor is developed with a toner. Next, as needed, a toner image thus formed is transferred onto a transfer material such as a paper sheet and then fixed thereon by various methods such as heating, pressurization or solvent fume.
In the field of such an image forming device, a digital full-color copying machine and a digital full-color printer have been put to practical use. A digital full-color copying machine produces a full-color image as follows. First, an original color image is subjected to color separation with blue, green and red filters; an electrostatic latent image corresponding to the original color image, which is composed of dots that are 20 μm to 70 μm in diameter, is developed with yellow, magenta, cyan and black toners; and a full-color image is famed using the subtractive color mixing effect.
Recently, there is an increasing demand for full-color images with high image quality and high resolution. Especially, to increase color reproducibility, it is hoped that an image can be printed in the same hue as ink printing.
As a color pigment for a yellow toner, for example, a disazo pigment as typified by C.I. Pigment Yellow 12, 13 and 17, and a monoazo pigment as typified by C.I. Pigment Yellow 74, 97 and 98 have been generally used.
Patent Literature 1 discloses a yellow toner comprising C.I. Pigment Yellow 214 and C.I. Pigment Yellow 139 as colorants. Patent Literature 1 describes that a toner excellent in fixability and color properties is obtained by interaction between the amino groups of C.I. Pigment Yellow 214 and the carbonyl groups of C.I. Pigment Yellow 139.
Patent Literature 2 discloses a yellow toner comprising C.I. Pigment Yellow 93 and C.I. Solvent Yellow 162 as colorants. Patent Literature 2 describes that by containing such colorants, a yellow image projected by an overhead projector (OHP) shows excellent transparency.
Patent Literature 3 discloses a yellow toner comprising a combination of, as yellow colorants, C.I. Pigment Yellow 155 with C.I. Pigment Yellow 110 or the like at a specific ratio. Patent Literature 3 describes that by using such colorants, an excellent resistance to hot offset can be obtained even when using heat-and-pressure fixing means in which any oil is not used or such an oil is used in a small quantity; both long-team storage stability in a high-temperature environment and low-temperature fixing performance can be achieved; and color tones of process inks (color reproduction ranges) can be covered.